Multi-modal markers, i.e. markers that can be detected by more than one modality, allow for the registration of images by one modality to images from another modality. Important examples are quantitative images of the distribution of magnetic tracer material, which are obtained by MPI (Magnetic Particle Imaging) and images of the anatomy, which can be obtained by MRI (Magnetic Resonance Imaging). In the case of pre-clinical studies that involve the examination of specimen or for the body part of a patient that is fixed during imaging to avoid movements, the registration of holder devices is also desired, as it can be assumed that the object or specimen is in a fixed relation to the holder device. Thus, the images of the object or specimen are also registered if the holder device is registered, assuming that there is no motion of the specimen or body part with respect to the holder device.
MPI-visible fiducial markers allow robust registration of MPI images to anatomical images obtained using MRI or CT (Computed Tomography), and to other functional data obtained from hybrid PET-CT (PET=Positron Emission Tomography), PET-MR or SPECT-CT (SPECT=Single Photo Emission Computed Tomography) systems, if these markers are also visible in these other modality images.
Image registration between anatomical MRI or CT images, especially pre-clinical images with sub-0.1 mm spatial resolution, and MPI images—with their comparably low spatial resolution in the mm range—can be difficult when based on anatomical landmarks, because anatomical landmarks cannot easily and accurately be automatically detected in the respective modalities. Fiducial markers for this purpose should be easily detectable and localizable in the modality images, preferably with very high (sub-pixel (for 2D) or sub-voxel (for 3D)) accuracy.